In the pulp and paper industry, pulping refers to the process of converting wood chip feed stock into separate fibers by the chemical reaction between the lignin found in the wood chips and the active chemicals in a cooking liquor. This delignification process separates the wood cellulose fibers by breaking down the lignin. Lignin is a polymer of complex chemical structure which "cements" together the wood's cellulose fibers. The most prevalent method of delignification is by chemical means in which raw wood chips and chemicals are combined at a controlled pressure and temperature in a vessel known as a digester. While in the digester, the amount of lignin removed from the wood chips determines the product quality, the product yield, and the amount of energy consumed. Fluid drained from the digester during delignification contains lignin removed from the wood chips and is referred to in the industry as "black liquor". Black liquor is subsequently used to advantage during the pulping process as fuel in a boiler to produce process steam.
One common method of delignification presently used in pulp making is the kraft process. In this process, the wood chip feed stock is cooked with caustic soda and sodium sulfide, which removes most of the lignin without attacking the remaining cellulose fibers. When the remaining pulp of the kraft process is not passed through a bleaching process, it is used in cardboard or paper sack production. The dark color of this product is due to the remaining lignin in the fibers. However, if the final product of the process is to be a good-quality white paper, a bleaching step is introduced in the process using chlorine, chlorine dioxide, oxygen, ozone, or hydrogen peroxide as the bleaching agent. The bleaching dissolves the remaining lignin and renders white the remaining cellulose fibers. The amount of whiteness and the term or amount of time that the final paper product remains white are dependent on the remaining residual lignin in the cellulose fibers. It is, therefore, customary to test the lignin content of the pulp fibers and use this determination as a measure of the effectiveness of the ongoing bleaching operation.
Predicting the bleachability of the pulp in the prior art has been by the use of one or more of the several available tests such as the Permanganate Number (TAPPI method T-214), or the Kappa Number (TAPPI method T-236), or the Roe Chlorine Number (TAPPI method 202), etc. Each of these tests is designed to determine the quantity of lignin present in the pulp fibers as a group and provides an indication of the total bleach requirement (the oxidizing agent demand of the pulp) in the bleaching step. The most commonly used of these tests is the Kappa number, which refers to the amount of material remaining in the pulp after cooking that can be oxidized by a standard solution of potassium permanganate. The material is often equated with the lignin content of the fibers.
The Kappa number test, as well as the other tests noted, is most commonly carried out by laboratory analysis of hourly samples of the digester output (samples are typically obtained at the last stage of the brownstock washer). This requires extracting a representative sample of the pulp, separating the pulp fibers from the cooking liquor, drying the pulp to oven-dry conditions, re-suspending the fibers, and treating this new mixture with one or more special agents, all under strict laboratory conditions. The laboratory analysis of the residual lignin takes approximately one hour and, therefore, is a poor method for providing process control feedback and cannot be used for feedforward control. A number of automatic sampling and testing devices have been tried but they have been mostly unsuccessful in providing accurate long-term results and do not reduce the one-hour delay between process and measurement of the residual lignin.
Still other devices are known which use the ultraviolet (UV) fluorescence properties of lignin to measure the lignin concentration. Such testing systems require very dilute lignin solutions to be prepared prior to measurement and, therefore, are not suitable for in-situ, or real-time, testing. Other ultraviolet absorption testing methods have attempted to measure the residual lignin in wood pulp by sampling the pulp every few minutes, preparing and diluting the sample, and circulating the sample into a loop where the UV light absorption is measured over a prescribed time period and the pulp concentration is measured independently. Even though this system provides for a faster method of testing than that of the laboratory method, it is still off-line.
One system known which provides for in-situ lignin testing is taught by U.S. Pat. No. 5,486,915, issued on Jan. 23, 1996, to Jeffers et al. This lignin analyzer uses a fluorescence technique to measure lignin concentration in undiluted samples of wood pulp. This method and apparatus require the use of fairly complex detection methods that use the radiation of the wood pulp with excitation light in a specific wavelength (in the range of 337 nm) in order for the residual lignin in the pulp to emit fluorescence. A spectral distribution of the fluorescence emissions is then determined and output signals produced to a signal processor that quantifies the residual lignin in the pulp by either a wavelength centroid or band ratio method.
Even though this system provides for in-situ, real-time analysis of the lignin concentrations, it is more effective during the early stages of the bleaching process where greater concentrations of lignin are present. In the later stages of the bleaching process the lignin content is reduced appreciably, thereby diminishing the fluorescence emissions of the pulp. This method also does not lend itself to measuring the later stages of the pulp process involving the brightness processing of the pulp, important in the formation of quality paper products requiring a high brightness level. A balance of bleaching agent to brightness must be determined in order not to degrade the strength of the pulp, increase the cost of the bleaching operation, limit the exposure of personnel to toxic chemicals, and provide minimum impact on the environment.